Q Tomography of the Earth's Mantle using Long Period Waveforms

Yuancheng Gung and Barbara Romanowicz

Introduction

With the increasing reliability of the details in elastic global models, it may be possible, in the near future, to accurately predict elastic effects on the amplitudes of globally traveling long period surface and body waves, thereby gaining access to more accurate estimates of the lateral variations of Q in the earth.

In this preliminary study, we experiment with a waveform formalism, based on mode summation by path average method, applied to surface waves. Starting with the SH model, SAW12D (Li and Romanowicz, 1996), we invert iteratively for Q structure. The Q model is parameterized radially in terms of layers, and we progressively increase the maximum degree of the lateral expansion in spherical harmonics.

Data

To model the Q heterogeneity with waveform inversion progressively, only mantle Love waves of the first and second orbits (G1 and G2) are considered in this study. The SH accelerograms are low-pass filtered with a cutoff frequency of 1/80 Hz and a corner frequency 1/100 Hz.

11353 wave packets with good global coverage (Figure 30.1)are used in this study.

  

Figure 30.1: The data coverage density in this study, expressed as the logarithm of the total sampling length in degrees per cell

Results

Images of our degree 5 Q model in upper mantle layers are shown in Figure 30.2. The main features in this model are summarized in three parts:(1) The pattern in the uppermost mantle is well correlated with tectonic features. (2) the high Q in the Eurasian Plate and northern America and low Q in east pacific region are quite stable over different depths, while the low attenuation in northern mid-Atlantic region fades out at depth below 220km. (3) the amplitude of heterogeneity decreases with depth.

  

Figure 30.2: Images of Q model at four different depths. The lateral variations are expressed in terms of relative variations in Q with respect to the average in the layer: red and blue indicates regions of high and low attenuation, respectively. The distribution of hotspots according to ( Richard et al, 1988) is also shown, indicating its correlation with low Q.

References

Li, X.D. and B. Romanowicz, Global mantle shear-velocity model developed using nonlinear asymptotic coupling theory, Geophys. J. R. Astr. Soc., 101, 22,245-22,272, 1996.

Richard, M. A., B. H. Hager, and N. H. Sleep, Dynamically supported geoid highs over hotspots: observation and theory,J. Geophys. Res., 93 7690-7708.

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